Violently cyclic idle - MC engine

[Phil Payne]

Type 44 - 100 TW MC engine.

Occasionally the idle goes into a _VIOLENT_ cycle - quite agressively cycling between ca. 1050
rpm and 1500 rpm.  Cycle is around 1Hz.

Only code is 2312 (temperature sender) - I've tried three now and still get the code, but the
harness has been butchered at some time so I need to restore it.  I plan on diving in
tomorrow.

I've checked all the air mass sensor valves - decel shutoff, fuel frequency, carbon canister,
ISV, etc., and they all seem nominal.  I've got a boxfull of spares and even a complete spare
car to cannibalise.

I can sort this out from first principles just by taking it all to bits and putting it
together properly.  But has anyone seen this exact symptom?

-- 
  Phil Payne
  http://www.isham-research.com
  +44 7785 302 803